A print head for a thermal printer comprises a support plate supporting a line of heater points, each of which is constituted by an electrical resistance element capable of heating up under the Joule effect, and inserted in a current power conductor including an electronic switch.
The principle on which such an apparatus operates is simple. It consists firstly in causing a strip of temperature-sensitive paper to travel beneath the line of points and in contact therewith, and secondly in powering successive series of heater points so as to raise their temperatures for a short period of time to higher than their reaction temperature of the paper. Each point heated in this way thus leaves a mark on the paper, and the size of the mark depends on various factors such as the width of the heater point (taken transversely to the travel direction of the paper), the travel speed of the paper, the time the specified temperature is maintained at the point, . . . . One the next sequence, other points are heated up, while the preceding points cool down, and the mark resulting from a succession of such sequences forms the desired print.
In practice, numerous problems need to be solved, given the ever-increasing performance required of such printers.
One of these problems lies in regulating the temperature of each heater point. A certain amount of energy needs to be applied to a point in order to raise it to the temperature required for marking the paper. This energy for reaching the desired temperature is conveyed to the point in the form of a (rectangular) electrical pulse whose area depends both on the thermal inertia of the resistive material and on the starting temperature of the point to be heated. Unfortunately, the starting temperature is a function of factors that are independent of one another, and in particular it depends both on ambient temperature and on the time between two successive heating pulses. For any one point, these two factors are entirely random and variable, one depending on the printing that is being done while the other depends on the climatic and atmospheric conditions of the place where the printer is being used. It is therefore necessary to know the starting temperature of a point in order to be able to adjust the amount of energy that is to be delivered to the point so as to heat it sufficiently to obtain the desired result. Insufficient heating gives rise to no mark at all or to a mark having too little contrast. Excessive heating overdoes the marking and destroys fineness in the printing.
Present printers are fitted with a temperature sensor which is provided in the form of a thermistor type component. This component is disposed on the substrate at a considerable distance from the line of points, and as a result the response time of the component is of an order of magnitude (about 10 seconds) that is unacceptable, given the rate at which the temperature state of a line of points can change due to ever-increasing excitation rates. By way of example, the period of the pulses may be about 1 millisecond and this is to be compared with the 10 or 20 seconds response time of the temperature sensor, thus leading to regulation with a delay of 10,000 to 20,000 lines of print points.
An object of the present invention is to remedy this drawback by providing a thermal print head that includes means for providing a measurement of the temperature of the heater points with an extremely short response time, thus making it possible to provide fine regulation of the quantity of energy delivered to each heater point, thereby guaranteeing printing of constant quality both along the line and over time.
Under present conditions it is pointless and economically prohibitive to seek to sense the temperature directly of each point and at each heating pulse (thereby determining the energy to be delivered). It is nevertheless highly advantageous to be able to have the mean temperature of the line of points very quickly or at least the mean temperatures of various lengths of the line of hot points. First regulation should enable this mean temperature to be maintained at a given fraction of the reaction temperature of the paper. This improves print regularity. In addition, by discovering this mean temperature quickly, it is possible to associate its value with other operating parameters relating to the line of points, e.g. the excitation frequency of each of the points, thereby making it possible to draw conclusions suitable for "personalizing" the energy delivered to any given point, thus improving regulation.